If you go with TDI, there is no requirement to do Intro to Tech. If you have good in-water skills and 100 dives under your belt. Talk to the instructor you want to work with. Time in the water with the gear is the biggest priority, all the courses will not gloss over a lack-of-commitment. Its fine if you can only dive every other month, just note that no instructor, course or agency will make you look slick.
If following the TDI path, AN&DP is where most people start with 35-45m dives, 20-30mins of deco on a single deco mix, usually EAN50. Add Helitrox if you're feeling minted and can afford hyperoxic trimix. Extended Range is not a requirement for doing normoxic Trimix training, its an optional pathway. ER and Trimix both add a second deco tank of 100% O2. Both use the same eLearning and mostly the same skills, Trimix is far nicer to breathe at +50m and for me its the better choice.
Regarding gear config, BM is a solid choice and SM is its own skillset. Start in SM if you want to do SM, don't change over halfway without doing a reset to re-learn the instinctive physical skills. I've seen some superb SM tech divers, but the only divers I've had to assist were SM divers that lacked the skills and behaviours to tech dive in SM configuration.
Progress with a buddy if you can, or a sympathetic group. A mate of mine finished his Air Dil CCR course over the weekend and we did a few dives after his course where we just floated about at 12-18m and let him get a few hours feeling the machine and getting more intuitive with his senses and his reactions. It really helps, especially early on, if you can spend an entire dive on a sloping shore, ie sitting in 10m of water practicing holding 9m depth, letting the surge push you around, fiddling with your gear to get your trim just right so you can float at depth without moving. Practice your superman pose, kick and glide, practice subtle changes in depth without changing your bouyancy, how you breathe on deco stops, just keep refining your skills and getting smoother etc.
GUE guys are slick in the water, I would not argue its the gold standard. If its not for you then you can join the very large group that it also wasn't for without the smoking. You'll get great instruction and great learning materials with a different agency, plus more flexibility in how you plan and execute your dives. I'd love to do Fundies one day to gauge its place as the 'benchmark', but its impossible in my region and I have no interest in the rigidity of GUE. As a system it produces excellent divers and you would see many positives if you went that way. Personal experiences do vary, there are many positives to be had with other agencies.